When a water tap is turned on, water flows out continuously. It's easy to think that freshwater will always be plentiful, and the threat of an impending water crisis will rarely happen. However, freshwater is incredibly rare. A mere 0.014% of all water on earth is both fresh and easily accessible. As a result, some 1.1 billion people worldwide lack access to clean water, and every year a total of 3.1 million suffer and die from water-borne illnesses caused by inadequate sanitation. In the near future, two-thirds of the world's population may face water scarcity. In 1992, the United Nations General Assembly adopted a resolution by which 22 March of each year was declared World Day for Water. Statistically, each urban resident consumes 30-50 liters of water per day for flushing the toilet. When calculating on the basis of 7.5 billion population, the water consumed by the flush-toilets every day in China reaches 30 million tons (the storage water amount of the West Lake is 14 million tons), excluding the water consumption of flush-toilets in rural areas. In the prior art, a waste water container is arranged between every two adjacent floors, resulting in a huge amount of construction work and a high spatial occupation. As water leakage frequently occurs, the maintenance cost is high. Moreover, the intermediate water cannot be cross-floor utilized. In conclusion, it's urgent for those skilled in this field to develop a novel water conservation system for use in a building.